This invention is in the field of fluoropolymer resins, and deals with the stabilization of such resins having unstable end groups.
Melt-fabricable copolymers of tetrafluoroethylene (TFE) and various copolymerizable mononmers such as hexafluoropropylene (HFP) are well known, as are polymerization processes for making them. Bro and Sandt in often-cited U.S. Pat. No. 2,946,763 disclose an aqueous process for TFE/HFP copolymers using water-soluble free-radical initiator. As a consequence of the initiators used, TFE copolymers made by aqueous processes such as that of Bro and Sandt have unstable end groups, notably xe2x80x94COOH or salts thereof, that can decompose during subsequent processing and result in unacceptable bubbling in finished products.
Various processes for stabilizing the end groups of such polymers have been devised. Schreyer, for example, in U.S. Pat. No. 3,085,083 discloses a humid heat treatment process for improving the stability of such polymers by converting unstable carboxylate end groups to relatively stable xe2x80x94CF2H (hydride) end groups. Imbalzano and Kerbow in U.S. Pat. No. 4,743,658 disclose fluorine treatment of copolymers of TFE and perfluoro(alkyl vinyl ether) (PAVE) to reduce the population of unstable end groups to very low levels. Such polymer finishing steps are time-consuming and costly.
Morgan and Sloan in U.S. Pat. No. 4,626,587 disclose a high-shear thermo-mechanical process for reducing the backbone instability in TFE/HFP copolymers. It is disclosed that, if the polymer contains unstable end groups or has poor color after removal from the high-shear extruder, such problems can be eliminated by fluorination (contact with elemental fluorine). All of Examples 1-3 of Morgan and Sloan did in fact have poor color after backbone stabilization by extrusion, as indicated by low values (5-18) of the %G color parameter, and required fluorine treatment to improve %G to the 49-54 range. As mentioned above, fluorine treatment is costly.
Mallouk and Sandt in U.S. Pat. No. 2,955,099 disclose viscosity stabilized TFE/HFP interpolymers prepared according to the Bro and Sandt ""763 patent for which the viscosity stability is achieved by incorporation of cationic metal compound. Compounds of cationic metals such as potassium, caesium and rubidium, particularly in the form of salts of labile anions such as iodide and bromide, are said to be most active. Potassium perchlorate is said to be used with particular advantage to improve color since it functions in the dual role of oxidant and viscosity stabilizer. However, Mallouk and Sandt also disclose that compounds of cationic metal with anions such as carbonate and nitrate tend to cause inhomogeneities in the interpolymer and are therefore not suitable for use as viscosity stabilizers. As shown by the Reference Example below, TFE/HFP interpolymers prepared according to Bro and Sandt, additionally, are not suitable for use in the process of the present invention.
Gibbard in U.S. Pat. No. 5,180,803 discloses a method for the production of a melt-fabricable fluoropolymer, which method comprises, after preparing an aqueous dispersion of the fluoropolymer using dispersion polymerization with an initiator that yields carboxylic acid groups on the fluoropolymer, converting the carboxylic acid groups on the fluoropolymer in the aqueous dispersion to carboxylate anion groups using a base and then heating the so-modified fluoropolymer dispersion to a temperature of 190xc2x0-240xc2x0 C. to cause simultaneously (1) substantial removal of carboxylate anion groups, thereby to yield stable groups in their place, and (2) isolation of the fluoropolymer from the dispersion by coagulation of the fluoropolymer dispersion. Gibbard""s EXAMPLE 1 discloses stabilization and isolation of TFE/HFP copolymer manufactured by the process of Bro and Sandt by addition of 1 wt % of potassium hydroxide, based on dry polymer solids, followed by neutralization with nitric acid which would correspond to formation of about 1.6 wt % of KNO3, again based on the weight of dry TFE/HFP copolymer solids. One skilled in the art would then expect about 1700-4000 ppm of KNO3 to be in the copolymer resin after isolation and drying.
TFE/HFP copolymers of the prior art, such as mentioned above, prepared by the process of Bro and Sandt have a high degree of instability, with end group and backbone contributions. This instability may be characterized by measurement of a total unstable fraction (TUF), as described herein. Polymers of the Bro and Sandt type generally have TUF of at least 0.5% when melt viscosity is in the range of 1-10xc3x97103 Paxc2x7s.
A new polymerization process for TFE/HFP/PAVE copolymers, disclosed in European Patent Application 96-112948 (Publication 0 789 038), yields copolymer having high stability, characterized by a total unstable fraction of no more than 0.3%. In this process, copolymerizing is carried out with chain transfer agent present and with initiator present in an amount effective to initiate no more than half of the copolymer molecules made. The combined use of increased CTA and reduced initiator results in a TFE/HFP copolymer having improved total stability, i.e., the combination of reduced backbone instability and a reduced xe2x80x94COOH end group population. The resultant TFE/HFP copolymer can be used for many purposes without special stabilization finishing steps. However, if very high stability or complete absence of discoloration is required, some additional treatment such as fluorination would be necessary.
The problem of obtaining TFE/HFP copolymer and other melt-fabricable fluoropolymer resins of even higher stability and/or good color to permit commercial use without a costly stabilization finishing procedure is solved by a convenient and inexpensive process. The process for stabilizing and whitening melt-fabricable fluoropolymer resin having total unstable fraction, as defined herein, of no more than 0.3%, comprises extruding said resin in the presence of alkali metal nitrate to obtain said fluoropolymer resin having improved color and/or stability. The nitrate can be introduced into aqueous polymerization of the fluoropolymer, before isolation of the fluoropolymer solids from the aqueous dispersion, or after isolation of the resin from the polymerization medium and prior to melt extrusion. Preferred fluoropolymers for treatment by this process are tetrafluoroethylene copolymers.
It has been discovered that a small amount of alkali metal nitrate present during melt extrusion of fluoropolymer resin having a low level of instability as delivered to the extruder results in improved stability and/or color.
The fluoropolymers for which the process of this invention is suitable are melt-fabricable. As such, they generally have melt viscosity (MV) in the range of 0.5-50xc3x97103 Paxc2x7s, though MV outside this range is known. MV of 1-20xc3x97103 Paxc2x7s is preferred.
Preferred fluoropolymers for the process of this invention include melt-fabricable copolymers of TFE with a sufficient amount of at least one fluorinated comonomer to reduce the melting point of the copolymer substantially below that of homopolymer polytetrafluoroethylene, e.g., to a melting point of no more than about 320xc2x0 C. Comonomers that can be copolymerized with TFE include, for example, HFP and fluorinated vinyl ethers (FVE) of the formula CF2xe2x95x90CFOR or CF2xe2x95x90CFxe2x80x94ORxe2x80x2xe2x80x94OR wherein xe2x80x94R and xe2x80x94Rxe2x80x2xe2x80x94 are independently completely-fluorinated or partially-fluorinated linear or branched alkyl and alkylene groups containing 1-8 carbon atoms. Preferred xe2x80x94R groups contain 1-4 carbon atoms, while preferred xe2x80x94Rxe2x80x2xe2x80x94 groups contain 2-4 carbon atoms. FVE of the formula CF2xe2x95x90CFOR are preferred, particularly perfluoro(alkyl vinyl ether) (PAVE). Preferred TFE copolymers include TFE/PAVE and TFE/HFP/PAVE. Preferred PAVE include perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(propyl vinyl ether) (PPVE). As one skilled in the art will recognize, the amount of comonomer used will vary with the properties desired and the type of comonomer, which in turn may result in different melting points. This is illustrated, for example, by typical TFE/HFP (FEP) and TFE/PAVE (PFA) copolymer resins as described, respectively, in ASTM Standard Specifications D-2116 and D-3307.
Fluoropolymers subjected to the process of this invention can be characterized by weight loss as a result of controlled high temperature exposure. The method employed is designed to measure total unstable fraction (TUF). As described below, TUF involves the difference between weight losses measured after exposure for different times at high temperature in dry nitrogen. TUF is used herein to characterize the fluoropolymer subjected to the proecess, i.e., the feed to extrusion, as well as to illustrate the effect of this invention. TUF for melt-fabricable fluoropolymer used in the process of this invention is no more than 0.3%, preferably no more than 0.2%.
Fluoropolymer having TUF of no more that 0.3% can be prepared by various methods. Generally, it is desirable to prepare such fluoropolymer directly in polymerization. This can be done, for example, in aqueous polymerization of TFE/PAVE copolymer by using using chain transfer agent (CTA) that yields relatively stable end groups, or in aqueous polymerization of TFE/HFP/PAVE copolymer using a balance of CTA along with chosen HFP and PAVE concentrations, or by use of initiators that yield stable end groups in non-aqueous or in suspension polymerizations. One skilled in the art will recognize that fluoropolymer suitable as feed for the process of this invention, i.e., having TUF of no more than 0.3%, can be prepared from fluoropolymer initially less stable by subjecting this initial fluoropolymer to a preliminary stabilization process to reduce TUF to 0.3% or less. However, it is generally desirable to avoid such preliminary stabilization, if possible.
After subjecting melt-fabricable fluoropolymer resin to the process of this invention, the fluoropolymer will have improved stability or improved color, or both. Improved stability can be indicated by a reduced population of unstable end groups, and color may be improved. The color of fluoropolymer resin extruded according to the process of this invention is generally good, usually having color parameters %Gxe2x89xa735, YIxe2x89xa610, and WIxe2x89xa735, preferably having color parameters %Gxe2x89xa740, YIxe2x89xa66, and WIxe2x89xa740. More preferably, %Gxe2x89xa745, YIxe2x89xa60, and WIxe2x89xa750.
As used herein, xe2x80x9cextrusion in the presence of alkali metal nitratexe2x80x9d signifies that nitrate is introduced into the fluoropolymer resin prior to melt extrusion. The nitrate can be introduced into the resin during any process step prior to extrusion, including polymerization, isolation of polymer solids from the polymerization medium, drying, and preparation of extruder feed. For fluoropolymer prepared by aqueous dispersion polymerization, a preferred point for introducing the nitrate is during isolation. Preferred points for introducing alkali metal nitrate also include during polymerization because of the uniform distribution of nitrate that results. Contrary to expectation, polymerization is not adversely affected by the presence of nitrate. While nitrate can be introduced at more than one point in the process, e.g., during polymerization and again in the extruder feed, a single addition will normally be employed. The nitrate can be introduced as a solid or as an aqueous solution, as desired and as appropriate to the process step chosen for introduction. When introduced into aqueous polymerization, it is convenient to pump a solution into the reactor. On the other hand, one would normally introduce alkali metal nitrate into extruder feed as a dry solid blended with the resin or co-fed with the resin. It is also possible to inject a nitrate solution into the extruder if the extruder is of appropriate design. The alkali metal nitrate, of course, can be formed in situ, e.g., by introducing appropriate acid and base in an aqueous process step, but it is simpler to introduce the nitrate as such. Alkali metal nitrate will usually be present at the first extrusion of the fluoropolymer resin, a situation that necessarily follows if the nitrate is introduced during a wet process step. The first extrusion is normally part of the fluoropolymer resin finishing procedure, to prepare the resin for commercial use in a form such as pellets or cubes. However, one skilled in the art will recognize that such first extrusion could result in fabrication of a finished article, e.g., film or tubing. Likewise, one will recognize the possibility of adding alkali metal nitrate to resin that has been previously extruded, either with or without nitrate present.
Any alkali metal nitrate can be used in the practice of the present invention. Preferably, the alkali metal is potassium or sodium. One skilled in the art will recognize that a combination of alkali metal nitrates can be used.
Preferably, the amount of alkali metal nitrate used in the process of this invention is in the range of 20-500 ppm, most preferably 50-250 ppm, by weight based on the weight of dry fluoropolymer resin feed to extrusion, with the amount of alkali metal nitrate normalized to the molecular weight of potassium nitrate. Thus, if introduced during polymerization or a wet isolation step, the amount of nitrate added would be increased to compensate for the amount of salt that is carried off by the separated water. If nitrate is introduced during more than one process step, the amounts recited above apply to the combined amounts as calculated for each introduction. Amounts of alkali metal nitrate greater than 500 ppm can have a beneficial effect on color but an adverse effect on high-temperature stability of the fluoropolymer resin, e.g., as indicated by MV. Thus, as the amount of alkali metal nitrate is increased above 500 ppm, improvement in color may be at some sacrifice in resin stability. In some applications, some reduction in MV is tolerable. To minimize this adverse effect, the amount of alkali metal nitrate will be no greater than 1000 ppm.
Methods of melt-extrusion that can be used in the process of this invention include methods known in the art for use with fluoropolymers. Extruders that can be used include twin-screw and single-screw extruders. Twin-screw extruders such as those supplied by Werner and Pfleiderer are preferred. One skilled in the art will recognize such alternative possibilities as compounding fluoropolymer resin in the presence of nitrate in high intensity mixers, such as a Henschel mixer, followed by melt extrusion of the compounded resin. This is considered to be melt-extrusion in the presence of nitrate and within the scope of the present invention.